Multimode laser diodes are also known as broad area lasers (BALs). Broad area laser diodes are edge-emitting laser diodes in which the emitting region has a strongly asymmetric shape in the form of a broad stripe. State of the art high-power laser diode packages collimate broad area lasers using a fast axis collimator and a slow axis collimator. These laser diode packages use fast axis collimator lenses having focal lengths on the order of 0.15 mm-5 mm, with higher performance packages having shorter fast axis collimator lens focal lengths, such as for example, a 0.32 mm focal length. Additionally, most laser diode packages use a plano-convex slow axis collimator lens having a focal length of between 5 mm and 15 mm, which is dictated by the slow-axis brightness of the laser diode. Conservation of etendue drives the residual divergence of the collimated beam, and minimizing overall package footprint of the laser diode package is a longstanding objective.
Generally, as the broad area laser increases in width, for example, from a 95 μm emitter to a 250 μm emitter, the reduced brightness of the wider emitter dictates more asymmetry in the optical system. Minimizing the asymmetry may be accomplished by increasing the focal length of the slow axis collimator, but doing so would negatively affect the overall physical footprint of the laser diode package. Current trends are, however, to reduce the size/volume/mass of the laser diode package, not to increase it. A simplified example is one in which the slow axis focal length in currently available packages occupies about 20 mm of the available overall package width of approximately 70 mm (i.e., about 28%). Devising a method to reduce the focal length of the slow axis collimator, and therefore its back focal length, would result in reduction of the package footprint/volume/mass.